I much prefer the front tie-downs to provide both lateral and vertical support - meaning pop the hood, tie webbing loops, then close the hood with the loop sticking out. A long boat will want to twist sideways in gusts, so two bits of parachute cord holding it from side-to-side really helps.
Originally Posted by fc321
I have taken canoes on car tops for thousands and thousands of miles. One line on the front and none on the back will probably work, but given the consequences of failure (possible death to the person behind), I don't risk it. I put a minimum of 2 lines on the front, cam straps or rope to hold to the roof rack, and one rope on the back.
View my fuel log 2014 Mirage SE 1.2 manual: 45.0 mpg (US) ... 19.1 km/L ... 5.2 L/100 km ... 54.1 mpg (Imp)